Okay. Last period on 11th June, which is just over 2 months ago... If you were pregnant I'm sure you would have had some sort of warning by now. Morning sickness, sore breasts and all of the other tell-tale signs.
If you are concerned visit your doctor. You don't mention where you're from, but I know in the UK the NHS have some excellent clinics for teenagers who find themselves in trouble. They can give you another test and help you whatever the outcome.
Missed periods can be down to stress, lack of nourishment or certain medical conditions (such as endometriosis), so don't worry yourself too much until you get a definite answer. And don't sit there worrying - get an appointment with your doctor.